This invention relates to multichannel communication systems including power amplifiers, and more particularly to distortion correction in a multichannel system.
Modern communication satellites provide several broadband repeater channels, which receive from an Earth station a plurality of signals within a cumulative frequency band, process the received signals, as by low-noise amplification, filtration and block conversion to another frequency, for retransmission to the same or another location. While the description of the invention is couched in terms of a communications satellite, the invention may also be used for terrestrial or other uses.
FIG. 1a illustrates, in simplified block diagram form, a satellite body 6 in accordance with the prior art, upon which are mounted a polarizing grid arrangement 8, vertically polarized receiving antenna 12V and horizontally polarized receiving antenna 12H. Receiving antennas 12V and 12H are coupled to vertical and horizontal signal processing arrangements 10V and 10H, respectively, located within body 6. Signal processing arrangements 10V and 10H process the received signals to produce signals to be retransmitted, which are broadcast by transmitting antennas 32V and 32H, respectively. Signal processing arrangement 10H is similar to vertical processing unit 10V, so only processing unit 10V is described.
The nature of the signals arriving at the satellite may be understood by reference to FIG. 1b. The vertically-polarized signals arriving at antenna 12V by way of polarizing grid 8 includes a plurality of signals centered at different frequencies f1, f2, f3. The amplitude spectra of various signals are designated V1, V2, V3 in FIG. 1b. Some of the signals arriving with horizontal polarization (at antenna 12H) are illustrated (in dashed lines) as H1, H2 in FIG. 1b. In a typical satellite system, there may be as many as 10 or more vertical (V) and 10 or more horizontal (H) channels, with their frequencies of operation interleaved as shown in FIG. 1b. The bandwidth of a signal such as signal V2 may be sufficient to carry a television channel, or more. Thus, the bandwidth of a signal such as V2 may be 6 MHz or more. Vertical processing channel 10V of FIG. 1a may, as a consequence, receive 10 or more signals V1, V2, V3 . . . V.sub.N, each six or more MHz wide, which are separated from each other by a like amount. Thus, the total frequency bandwidth occupied by the vertical signals may be 120 MHz or more, calculated as [10(v)+10(H)].times.6. The center frequency of the 120 MHz band may be, for example, at 14 GHz.
The 10 or more vertical signals V1, V2 . . . received by antenna 12V of FIG. 1a are coupled to an input filter 14 of channel 10V, for reducing noise and preventing interference. Filter 14 is a bandpass filter with a bandwidth substantially equal to the total bandwidth of the vertical signals. The filtered signals are coupled from input filter 14 to a low noise amplifier (not illustrated) if required, and then to a block converter including a mixer 16 and a local oscillator 18. The frequency of local oscillator 18 is selected to convert the 14 GHz center frequency to some other center frequency, such as 12 GHz. The downconverted 12 GHz signals are applied over a transmission path 20 to a multiplexing (MUX) filter 22. Multiplexing filter 22 separates signals V1, V2, V3 . . . from each other in accordance with their frequencies. Multiplexing filter 22 is the starting point for a plurality of separate channels designated generally as 1, 2, . . . 3, 4. If there are 10 vertical signals V1, V2, V3 . . . , then the number of channels in signal processor 10V is also 10. The signal in each of channels 1, 2, . . . 3, 4 is one of the signals V1, V2, . . . . In effect, filter 22 is a source of signals at a plurality of different frequencies, driving a like plurality of separate channels.
In general, the signals on channels 1, 2, . . . 3, 4 in FIG. 1a are amplified, the distortion generated due to the amplification is compensated, and the amplified and distortion corrected signals are applied to a combiner or demultiplexer 30, which may be a filter similar to filter 22 operated in reverse, or it might be a group of hybrid combiners which do not discriminate based upon frequency. The combined signals at the output of combiner 30 are applied to a transmitting antenna 32V for transmission back to an Earth station, or possibly to another satellite.
System considerations such as the signal strength of the signal available at the satellite, the receiving antenna gain, and the transmitting antenna gain and field strength required to reach the ground station establish the overall power gain which must be provided in each channel between receiving antenna 12V and transmitting antenna 32V.
Within any channel 1, 2, . . . 3, 4 of FIG. 1a, the signal is processed by the cascade of a driver amplifier (DA) 34, a distortion linearizer such as a predistortion equalizer (PDL) 36, and a power amplifier or final amplifier (FA) 38. For example, as illustrated in FIG. 1a, the cascade of a DA 34.sup.1, PDL 36.sup.1 and FA 38.sup.1 processes the channel 1 signals, and a DA 34.sup.2, PDL 36.sup.2, and FA 38.sup.2 amplifies the signals for channel 2. As illustrated in FIG. 1a, an additional cascade of a DA 34.sup.5, PDL 36.sup.5, and FA 38.sup.5 is connected in cascade, to define a supernumerary "channel" designated 5. Channel 5 is not connected for handling signal, but instead represents a reserve cascade which may be substituted into any of the other channels in which the cascade may become defective. To this end, connection between input filter 22 and the inputs of the various channel cascades 34, 36, 38 is provided by means of an input switch arrangement designated 24, and connection between the outputs of final amplifiers 38 and combiner 30 is provided by an output switch arrangement designated as 28. A switch control arrangement illustrated as 26 gangs the input and output switches for simultaneous operation, and responds to signals in response to evidence of failure, generated on the ground or autonomously by control circuits within the spacecraft itself. Thus, in the event that the cascade of DA 34.sup.1, PDL 36.sup.1 1, and FA 38.sup.1 fails completely or becomes degraded, the reserve cascade including DA 34.sup.5, PDL 36.sup.5, and FA 38.sup.5 can be substituted therefor, with the cascade of DA 34.sup.1, PDL 36.sup.1, and FA 38.sup.1 being removed from on-line use. Naturally, additional redundant units may be provided, and if the number of failures should exceed the number of redundant units, the switching arrangement including 24, 26 and 28 may move operable cascades from lower-priority uses to higher-priority uses. In order to be switchable to obtain this level of reliability, each cascade must have an instantaneous frequency bandwidth covering the cumulative or total bandwidth of the vertical signals V1, V2, V3, . . . .
An improved communications system is desired.